Electron Trapping Process Research Articles

Sunlight‐Activated Eu2+‐Doped Red Persistent Luminescence Material for Night‐Vision Signage, Anti‐Counterfeiting and Location Detection

AbstractPersistent luminescence materials have been widely studied due to their excellent optical properties. However, they can only be activated by UV light; in other words, the charging method for these phosphors is limited. In fact, materials that can continuously emit light when exposed to sunlight are highly sought after for their potential to reduce energy consumption. Warm‐colored persistent luminescence materials are particularly useful for information storage, security marking, and preventing counterfeiting. Nevertheless, creating effective warm‐color persistent luminescence materials that are activated by sunlight remains a challenging task. To address this issue, novel daylight‐activated red persistent luminescence materials, Gd3‐xCax‐0.02GaO6:0.02Eu2+ (x = 0.3 ‐ 0.7) are explored. The material can be effectively activated by sunlight under all weather conditions, as well as by bright light from indoor lamps, mobile phone screens, and any form of visible light. Moreover, it possesses excellent water resistance. The process of electron trapping and release within the material is investigated through thermoluminescence experiments, photoluminescence spectra, and persistent luminescence spectra. Importantly, this phosphor has been demonstrated for various applications, including night vision marking, anti‐counterfeiting, optical information storage, and positioning, among others.

Characterization of program and erase speed of memory capacitors with nanolaminated HfO2/Al2O3 stacks for application in non-volatile memories

The programming and erase performance of atomic layer deposited HfO2/Al2O3 memory stacks is investigated depending on the thickness of the tunnel oxide and the annealing of the structures. The as-grown stacks require applying of voltage pulses with duration of about 10-3 s for a measurable electron trapping (programming). The time dependence of the electron trapping process is quite sharp initially - the increase of the charging time from 4×10-4 s to 4×10-3 s provides more than 80% of the saturation value of the trapped charge obtained at durations above 1 s. The annealing increases the time needed for the onset of the electron trapping. For the chosen voltage amplitudes the programming characteristics does not depend on the tunnel oxide thickness. The electron detrapping (erase) is observed even at durations of ~10-7 s, but with a low efficiency. The erase characteristics is more gradual then the program one, and shows some dependence on tunnel oxide and annealing. The complete erase is achieved for times comparable to those for programming the cell. Erase pulses with duration > 10-2 s results to accumulation of positive charge in the capacitors.

Profound influence of surface trap states on the utilization of charge carriers in CdS photoanodes

In CdS photoanodes, the decoration of surface sulfur vacancies by CTAB passivates the electron trapping process, resulting in improved photoelectrochemical water oxidation performance.

Noble Metal Nanoparticle-Loaded Porphyrin Hexagonal Submicrowires Composites (M-HW): Photocatalytic Synthesis and Enhanced Photocatalytic Activity.

Surface plasmon resonance (SPR) photocatalysts have attracted considerable attention because of their strong absorption capacity of visible light and enhanced photogenic carrier separation efficiency. However, the separate production of metal nanoparticles (NPs) and semiconductors limits the photogenic charge transfer. As one of the most promising organic photocatalysts, porphyrin self-assemblies with a long-range ordered structure-enhance electron transfer. In this study, plasmonic noble metal-based porphyrin hexagonal submicrowires composites (M-HW) loaded with platinum (Pt), silver (Ag), gold (Au), and palladium (Pd) NPs were synthesized through a simple in situ photocatalytic method. Homogeneous and uniformly distributed metal particles on the M-HW composites enhanced the catalytic or chemical properties of the organic functional nanostructures. Under the same loading of metal NPs, the methyl orange photocatalytic degradation efficiency of Ag-HW [kAg-HW (0.043 min-1)] composite was three times higher than that of HW, followed by Pt-HW [kPt-HW (0.0417 min-1)], Au-HW [kAu-HW (0.0312 min-1)], and Pd-HW [kPd-HW (0.0198 min-1)]. However, the rhodamine B (RhB) and eosin B photocatalytic degradations of Pt-HW were 4 times and 2.6 times those of HW, respectively. Finally, the SPR-induced electron injection, trapping, and recombination processes of the M-HW system were investigated. These results showed that M-HW plasmonic photocatalysts exhibited excellent photocatalytic performances, making them promising materials for photodegrading organic pollutants.

Near-Infrared Emission Material with Superlong Afterglow Performance and Its Multifunctional Applications.

A new near-infrared (NIR) emission material, BaSi1.5Ge2.5O9:Cr3+, with outstanding long afterglow properties is designed and synthesized successfully. Its structure, photoluminescence, and long afterglow emission features are studied in detail. Cr3+ occupies six-coordinated Ba2+ and Ge4+ sites in this system, which can emit characteristic broadband NIR light in the range of 700-1200 nm, and it has a long afterglow emission of more than 10 h. We calculated the band gap of the host at about 4.1 eV, the energy level distribution after Cr3+ doping, and the distribution of oxygen vacancies through density functional theory simulation, which theoretically proved the characteristic transition of Cr3+ and that the oxygen vacancies are crucial to form the defect energy levels. This is corroborated with the reflectance spectra, three-dimensional thermoluminescence spectra, and electron spin resonance (ESR) spectra. The oxygen-deficient environment favors the formation of oxygen vacancy defects and prevents the oxidation of Cr3+ to Cr6+, which are the key points for the luminescence and afterglow properties. A mechanistic model of defect formation and electron trapping and transport processes is successfully established. Finally, its multifunctional applications in information anticounterfeiting encryption, biological tissue penetration, and internal nondestructive testing and imaging are demonstrated.

Charge Carrier Recombination Dynamics in MAPb(BrxCl1-x)3 Single Crystals.

Understanding carrier recombination processes in MAPb(BrxCl1-x)3 crystals is essential for their photoelectrical applications. In this work, carrier recombination dynamics in MAPb(BrxCl1-x)3 single crystals were studied by steady-state photoluminescence (PL), time-resolved photoluminescence (TRPL), and time-resolved microwave photoconductivity (TRMC). By comparing TRPL and TRMC, we find TRPL of MAPb(BrxCl1-x)3 (x < 0.98) single crystals is dominated by a hole trapping process while the long-lived component of TRMC is dominated by an electron trapping process. We also find both electron and hole trapping rates of MAPb(BrxCl1-x)3 (x < 0.98) crystals decrease with an increase in Br content. A temperature-dependent PL study shows there are shallow trap states besides the deep level trap states in the MAPb(Br0.82Cl0.18)3 crystal. The activation energy for holes in shallow trap states detrapped into the valence band is ∼0.1 eV, while the activation energy for free holes to be trapped into deep trap states is ∼0.4 eV. This work provides insight into carrier recombination processes in MAPb(BrxCl1-x)3 single crystals.

Protons: Critical Species for Resistive Switching in Interface‐Type Memristors

AbstractInterface‐type (IT) resistive switching (RS) memories are promising for next generation memory and computing technologies owing to the filament‐free switching, high on/off ratio, low power consumption, and low spatial variability. Although the switching mechanisms of memristors have been widely studied in filament‐type devices, they are largely unknown in IT memristors. In this work, using the simple Au/Nb:SrTiO3 (Nb:STO) as a model Schottky system, it is identified that protons from moisture are key element in determining the RS characteristics in IT memristors. The Au/Nb:STO devices show typical Schottky interface controlled current–voltage (I–V) curves with a large on/off ratio under ambient conditions. Surprisingly, in a controlled environment without protons/moisture, the large I–V hysteresis collapses with the disappearance of a high resistance state (HRS) and the Schottky barrier. Once the devices are re‐exposed to a humid environment, the typical large I–V hysteresis can be recovered within hours as the HRS and Schottky interface are restored. The RS mechanism in Au/Nb:STO is attributed to the Schottky barrier modulation by a proton assisted electron trapping and detrapping process. This work highlights the important role of protons/moisture in the RS properties of IT memristors and provides fundamental insight for switching mechanisms in metal oxides‐based memory devices.

Charge Carrier Recombination Dynamics in MAPb(Br1−yIy)3 Single Crystals

Studying the carrier recombination process in MAPb(Br1−yIy)3 single crystals (SCs) is important for its application in the optoelectronic field. In this work, a series of MAPb(Br1−yIy)3 SCs with varied Br/I compositions have been studied. Steady-state photoluminescence (PL) spectra, time-resolved photoluminescence (TRPL) spectra and time-resolved microwave photoconductivity (TRMC) were used to understand the radiative and non-radiative recombination processes of MAPb(Br1−yIy)3 SCs. By comparing the dynamics of TRPL and TRMC, we conclude that the dynamics of TRPL is dominated by the electron trapping process, which is in accordance with the fast decay component of TRMC kinetics, whereas the slower decay component in TRMC is dominated by the hole trapping process. Moreover, we find both the electron and hole trapping rates in mixed-halide perovskite MAPb(Br1−yIy)3 (0 &lt; y &lt; 1) SCs are higher than that of mono-halide perovskite MAPbBr3 SCs and MAPbI3 SCs. This suggests mixed-halide crystals could introduce additional electron and hole trapping densities, which could be related to the fluctuation of Br/I compositions in the crystals. This work is helpful for understanding carrier recombination process in mixed-halide perovskite SCs.

Direct Evidence of Interchange Instabilities at Dipolarization Fronts

AbstractUsing the high‐resolution measurements from magnetospheric multiscale mission, here we present the observational evidence of kinetic interchange instabilities (ICIs) at dipolarization fronts (DFs). Specifically, we present the observation of two DFs caused by ICI, and discover the sub‐ion scale magnetic depression caused by kinetic‐scale ICI at the second DF. By using multi‐spacecraft observation, the typical process inside the magnetic hole (MH)—by electron trapping process—is clearly shown in the magnetic depression, evidencing that the depression is a sub‐ion MH generated at DF by kinetic ICI. Based on observations and Minimum Variance Analysis (MVA) analysis, we present a two‐dimensional image of the ICI in the nonlinear stage. This observational study demonstrates the kinetic ICI at DFs, confirming the existence of MH generated by ICI at DF, and improves the understanding of dynamics at DFs.

High energy electron beam generation during interaction of a laser accelerated proton beam with a gas-discharge plasma

The study of the interaction between ion beam and plasma is very important to the areas of inertial fusion energy and high energy density physics. With detailed one-dimensional electromagnetic particle-in-cell simulations, we investigate here the interaction of a laser-accelerated proton beam assuming an ideal monoenergetic beam with a gas-discharge plasma. After the saturation stage of the two-stream instability excited by the proton beam, significant high energy electrons are observed, with maximum energy approaching 2 MeV, and a new two-stream instability occurs between the high energy electrons and background electrons. The trajectories of plasma electrons are studied, showing the process of electron trapping and de-trapping from the wakefield.

Stretchable, Breathable, and Stable Lead-Free Perovskite/Polymer Nanofiber Composite for Hybrid Triboelectric and Piezoelectric Energy Harvesting.

Halide-perovskite-based mechanical energy harvesters display excellent electrical output due to their unique ferroelectricity and dielectricity. However, their high toxicity and moisture sensitivity impede their practical applications. Herein, a stretchable, breathable, and stable nanofiber composite (LPPS-NFC) is fabricated through electrospinning of lead-free perovskite/poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and styrene-ethylene-butylene-styrene (SEBS). The Cs3 Bi2 Br9 perovskites serve as efficient electron acceptors and local nucleating agents for the crystallization of polymer chains, thereby enhancing the electron-trapping capacity and polar crystalline phase in LPPS-NFC. The excellent energy level matching between Cs3 Bi2 Br9 and PVDF-HFP boosts the electron transfer efficiency and reduces the charge loss, thereby promoting the electron-trapping process. Consequently, this LPPS-NFC-based energy harvester displays an excellent electrical output (400V, 1.63 µA cm-2 , and 2.34 W m-2 ), setting a record of the output voltage among halide-perovskite-based nanogenerators. The LPPS-NFC also exhibits excellent stretchability, waterproofness, and breathability, enabling the fabrication of robust wearable devices that convert mechanical energy from different biomechanical motions into electrical power to drive common electronic devices. The LPPS-NFC-based energy harvesters also endure extreme mechanical deformations (washing, folding, and crumpling) without performance degradation, and maintain stable electrical output up to 5 months, demonstrating their promising potential for use as smart textiles and wearable power sources.

Hidden surface photovoltages revealed by pump probe KPFM

In this work, we use pump-probe Kelvin probe force microscopy (pp-KPFM) in combination with non-contact atomic force microscopy (nc-AFM) under ultrahigh vacuum, to investigate the nature of the light-induced surface potential dynamics in alumina-passivated crystalline silicon, and in an organic bulk heterojunction thin film based on the PTB7-PC71BM tandem. In both cases, we demonstrate that it is possible to identify and separate the contributions of two different kinds of photo-induced charge distributions that give rise to potential shifts with opposite polarities, each characterized by different dynamics. The data acquired on the passivated crystalline silicon are shown to be fully consistent with the band-bending at the silicon-oxide interface, and with electron trapping processes in acceptors states and in the passivation layer. The full sequence of events that follow the electron–hole generation can be observed on the pp-KPFM curves, i.e. the carriers spatial separation and hole accumulation in the space charge area, the electron trapping, the electron–hole recombination, and finally the electron trap-release. Two dimensional dynamical maps of the organic blend photo-response are obtained by recording the pump-probe KPFM curves in data cube mode, and by implementing a specific batch processing protocol. Sample areas displaying an extra positive SPV component characterized by decay time-constants of a few tens of microseconds are thus revealed, and are tentatively attributed to specific interfaces formed between a polymer-enriched skin layer and recessed acceptor aggregates. Decay time constant images of the negative SPV component confirm that the acceptor clusters act as electron-trapping centres. Whatever the photovoltaic technology, our results exemplify how some of the SPV components may remain completely hidden to conventional SPV imaging by KPFM, with possible consequences in terms of photo-response misinterpretation. This work furthermore highlights the need of implementing time-resolved techniques that can provide a quantitative measurement of the time-resolved potential.

Modifying the Electron-Trapping Process at the BiVO4 Surface States via the TiO2 Overlayer for Enhanced Water Oxidation.

BiVO4 is one of the most promising photoanode candidates to achieve high-efficiency water splitting. However, overwhelming charge recombination at the interface limits its water oxidation activity. In this study, we show that the water oxidation activity of the BiVO4 photoanode is significantly boosted by the TiO2 overlayer prepared by atomic layer deposition. With a TiO2 overlayer of an optimized thickness, the photocurrent at 1.23 VRHE increased from 0.64 to 1.1 mA·cm-2 under front illumination corresponding to 72% enhancement. We attribute this substantial improvement to enhanced charge separation and suppression of surface recombination due to surface-state passivation. We provide direct evidence via transient photocurrent measurements that the TiO2 overlayer significantly decreases the photogenerated electron-trapping process at the BiVO4 surface. Electron-trapping passivation leads to enhanced electron photoconductivity, which results in higher photocurrent enhancement under front illumination rather than back illumination. This feature can be particularly useful for wireless tandem devices for water splitting as the higher band gap photoanodes are typically utilized with front illumination in such configurations. Even though the electron-trapping process is eliminated completely at higher TiO2 overlayer thicknesses, the charge-transfer resistance at the surface also increases significantly, resulting in a diminished photocurrent. We demonstrate that the ultrathin TiO2 overlayer can be used to fine tune the surface properties of BiVO4 and may be used for similar purposes for other photoelectrode systems and other photoelectrocatalytic reactions.

Flexible, ultra-stable and color tunable fluorescent films based on all inorganic perovskite quantum dots embedded in polymer

Colloidal perovskite CsPbX3 (X = Cl, Br, I) nanocrystals (CPNCs)/polymers composites have attracted extensive attention due to their potential to be developed as flexible phosphor films for lighting applications. However, to maintain high quantum efficiency and photo stability of CPNCs in such composites remains a daunting challenge. Here, we have demonstrated a layered composite structure consisting of CPNCs and polydimethylsiloxane (PDMS) with multi-color emission and long-term stability. By tuning the molar ratio between CsPbCl1.58Br1.42 and CsPbBr1.35I1.65, flexible fluorescent films as down-converter layers with a high luminescent efficiency and a controllable color temperature spanning from 3194 K to 5901 K have been demonstrated. Furthermore, due to embedding inside such composites, the quantum efficiency of CPNCs exhibited negligible changes during seven months in ambient conditions. The carrier dynamics based on time-resolved photoluminescence (PL) and transient absorption spectroscopy reveal that the hot electron tunneling and trapping process are significant in the composite film. This work provides a good understanding of CPNC materials in complex composite for the development of flexible, robust, color controllable fluorescent films for lighting applications.

Charge Separation Dynamics in CdSe/CdS Core/Shell Nanoplatelets Addressed by Coherent Electron Spin Precession.

We investigate the charge separation dynamics provided by carrier surface trapping in CdSe/CdS core/shell nanoplatelets by means of a three-laser-beam pump-orientation-probe technique, detecting the electron spin coherence at room temperature. Signals with two Larmor precession frequencies are found, which strongly differ in their dynamical characteristics and dependencies on pump power and shell thickness. The electron trapping process occurs on a time scale of about 10 ns, and the charge separation induced thereby has a long lifetime of up to hundreds of microseconds. On the other hand, the hole trapping requires times from subpicoseconds to hundreds of picoseconds, and the induced charge separation has a lifetime of a few nanoseconds. With increasing CdS shell thickness the hole trapping vanishes, while the electron trapping is still detectable. These findings have important implications for understanding the photophysical processes of nanoplatelets and other colloidal nanostructures.

Influence of the photoionization cross-section on the OSL signal of LiF: A theoretical and experimental approach

The understanding of the electron trapping and recombination processes is the key to successful applications of materials in ionizing radiation dosimetry, which unfortunately are not well known yet. In this work, the influence of the photoionization cross-section (σ) was investigated on the optically stimulated luminescence (OSL) response of the lithium fluoride (LiF) compound. The σ has been predicted through the expression proposed by Lima–Batista–Couto. In addition, OSL measurements were performed using the continuous-wave stimulation mode (CW-OSL), with 60 s of light stimulation. The samples were submitted to 10 Gy and 15 Gy of absorbed dose. Phonon dispersion and density calculations are presented by using the Density Functional Perturbation Theory. The results, obtained by a combination of different methodologies, show that the low sensitivity of LiF to the light stimulus (λ = 470 nm) is due to the very low magnitude of σ at the used excitation wavelength. A comparison with Al2O3:C was also carried out, showing that the intensity and the pattern of the OSL curve decay are modified by the σ. These results play an important role in understanding the luminescent properties of this material, and they open a new opportunity to improve the light sensitivity of this detector.

Asymmetric Bipolar Injection in a Schottky-Metal/${p}$ -GaN/AlGaN/GaN Device Under Forward Bias

In this letter, we investigated the electroluminescence (EL) spectra and photovoltage spectrum of a Schottky-metal/p-GaN/AlGaN/GaN diode with a semitransparent anode electrode. By analyzing the bias-dependence of the EL spectra of the diode as well as the gate stress bias-/time-dependences of the threshold voltage shift of a p-GaN gate HEMT, the electron/hole injection sequence, electron trapping, hole-assisted de-trapping, and optical pumping processes in the p-GaN gate region are identified. The band diagrams of the Schottky-metal/p-GaN/AlGaN/GaN heterostructure under different forward bias voltages are proposed.

Unified View of Nonlinear Wave Structures Associated with Whistler-Mode Chorus.

A range of nonlinear wave structures, including Langmuir waves, unipolar electric fields, and bipolar electric fields, are often observed in association with whistler-mode chorus waves in near-Earth space. We demonstrate that the three seemingly different nonlinear wave structures originate from the same nonlinear electron trapping process by whistler-mode chorus waves. The ratio of the Landau resonant velocity to the electron thermal velocity controls the type of nonlinear wave structures that will be generated.

Investigation of surface traps-induced current collapse phenomenon in AlGaN/GaN high electron mobility transistors with schottky gate structures

This paper reports on the studies of current collapse phenomenon induced by surface trapped charges during gate pulse switching in AlGaN/GaN heterostructure high-electron-mobility transistors. A physical-based model, taking into account the distribution features of the applied electric field along the surface of the device barrier layer near the drain-side gate corner, is proposed to analyse the electron trapping and de-trapping processes at the ionized donor-like traps during the device off-state or on-state process. Then the model is analysed and verified by TCAD simulation and laboratory measurement data. The morphology of the current collapse related AlGaN surface is investigated by SEM and AFM characterizations. The dynamic process and quantitative relationship between the electric field and trapped electron density are determined and analysed in detail. The spatial distributions of the trapped electrons and excess free electrons along AlGaN barrier surface are achieved by using the proposed physical model. The work provides a distinct perspective to understand and quantify the current collapse mechanism in AlGaN/GaN power devices, and it can also assist engineers for a better device design.

Impact of Shock Front Rippling and Self-reformation on the Electron Dynamics at Low-Mach-number Shocks

Abstract Electron dynamics at low-Mach-number collisionless shocks are investigated by using two-dimensional electromagnetic particle-in-cell simulations with various shock normal angles. We found: (1) The reflected ions and incident electrons at the shock front provide an effective mechanism for the quasi-electrostatic wave generation due to the charge-separation. A fraction of incident electrons can be effectively trapped and accelerated at the leading edge of the shock foot. (2) At quasi-perpendicular shocks, the electron trapping and reflection is nonuniform due to the shock rippling along the shock surface and is more likely to take place at some locations accompanied by intense reflected ion-beams. The electron trapping process has a periodical evolution over time due to the shock front self-reformation, which is controlled by ion dynamics. Thus, this is a cross-scale coupling phenomenon. (3) At quasi-parallel shocks, reflected ions can travel far back upstream. Consequently, quasi-electrostatic waves can be excited in the shock transition and the foreshock region. The electron trajectory analysis shows these waves can trap electrons at the foot region and reflect a fraction of them far back upstream. Simulation runs in this paper indicate that the micro-turbulence at the shock foot can provide a possible scenario for producing the reflected electron beam, which is a basic condition for the type II radio burst emission at low-Mach-number interplanetary shocks driven by Coronal Mass Ejections (CMEs).